I prefer Goo Gone since it's a little less harsh. I don't think either will mess up the glass on the windshield, but I'd keep it off the dash for sure.
In addition to the advice from Puckman, you might look at using a razor blade or a decal/glass scraper (easily found at a hardware store) to get the decal and all of the heavy adhesive deposits off. Once the major gunk is gone, go after it with the solvent and the mf towel.